The present invention relates, in general, to the field of magnetoresistive ("MR") spin valve ("SV") devices. More particularly, the present invention relates to a current perpendicular-to-the-plane ("CPP") spin valve device design, as well as an alternative differential sensing embodiment thereof, for use, for example, as a magnetic transducer or "head" for reading data signals encoded on a magnetic mass storage medium.
Magnetoresistive devices or heads exhibiting what is referred to as giant magnetoresistance ("GMR") are of current technological interest in an attempt to achieve higher areal density recording in magnetic computer mass storage disk drives and tapes. The GMR effect was first described by M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas in Phys. Rev. Lett. 61, 2472 (1988). Typically, the magnitude of the magnetoresistive ratio (".DELTA.R/R") for GMR materials exceeds that of anisotropic magnetoresistive ("AMR") materials, which are those generally in current use as magnetic read-transducers.
The spin valve effect is one known way to utilize GMR as described by B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit and D. Mauri, Phys. Rev. B 43, 1297 (1991). A typical spin valve MR device comprises two thin ferromagnetic layers (on the order of less than 100 .ANG.) separated by a nonmagnetic metal spacer (also on the order of less than 100 .ANG.). The magnetization of one ferromagnetic layer is allowed to move freely, whereas the other one is pinned by an adjacent antiferromagnetic or permanent magnetic layer. Essential to the operation of any type of GMR structure is the fact that the MR response is a function of the angle between two magnetization vectors corresponding to the sensing field.
A number of patents have previously described various device implementations utilizing the spin valve effect. See for example United States Patents No.: U.S. Pat. No. 5,159,513 to Dieny et al. for "Magnetoresistive Sensor Based on the Spin Valve Effect" issued Oct. 27, 1992; 5,206,590 to Dieny et al. for "Magnetoresistive Sensor Based on the Spin Valve Effect" issued Apr. 27, 1993; U.S. Pat. No. 5,287,238 to Baumgart et al. for "Dual Spin Valve Magnetoresistive Sensor" issued Feb. 15, 1994; and U.S. Pat. No. 5,301,079 to Cain et al. for "Current Biased Magnetoresistive Spin Valve Sensor" issued Apr. 5, 1994, all assigned to International Business Machines Corporation.
The stacked, orthogonal structures of the various device implementations described in these patents locate a lower ferromagnetic layer (on which the freely rotating magnetization vector resides) above the substrate but below the upper ferromagnetic layer which has its magnetization vector pinned by an adjacent antiferromagnetic pinning layer. Moreover, in all cases the sense current is shown to flow in the plane of the layers which comprise the spin valve structure, thereby limiting the MR response. Since the current density lies within the film plane of the magnetoresistive structure, this is known as a current-in-plane ("CIP") geometry.
In "A New Design for an Ultra-High Density Magnetic Recording Head Using a GMR Sensor In the CPP Mode"; Rottmayer, R. and Zhu, J.; IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995 there has been proposed a GMR multilayer read element within a write head gap that operates in the current perpendicular-to-the-plane mode and is biased by an exchange coupled soft film acting like a permanent magnet while distinguishing conventional MR and SV head designs. A multilayer read element consists of a repeated layer structure which is quite different from a spin valve type GMR transducer which comprises both pinned and freely rotating ferromagnetic layers as previously described. Moreover, the Rottmayer et al. paper contemplates the placement of the multilayer sensor in the write gap of a read/write head with a soft biasing magnet exchange coupled to an antiferromagnet to bias the sensor into a linear operating range. The placement of the read sensor within the write gap is a relatively unproven configuration due to the fact that the read sensor could suffer serious degradation through repeated exposure to fields generated during write current bursts.